The practical problem of finding the magnetic force tending to open a disconnecting switch has seldom been the subject of discussion in the past, but it is a very useful problem to solve. The result is expressed in concise form in formulas (20) and (21), and their derivation and the assumptions on which they are based are given. Curves are also presented in Figs. No. 3 and No. 4 from which the force may be found without using the formulas.